Normal enemies in the game can be killed with one shot to the head. Just aim at the head of any enemy with any gun and shoot to get a headshot. If they don't die then it wasn't a headshot. It's cumulative so you can do this at any point through-out the game and it'll just add to the total. Keep in mind there's a trophy for 5 consecutive headshot skills so it would be good to go for that first.